Heat transfer method



Dec. 16, 1958 H. T. BOOTH ET AL HEAT TRANSFER METHOD Filed March 25,1955 HEAT TRANSFER METHOD Harry '1. Booth, Frank E. Carroll, Jr., andKenneth 0. Parker, Dayton, Ohio, assignors to United Aircraft Products,llnc., Dayton, Ohio, a corporation of Ohio Application March 25, 1955,Serial No. 496,815

2 Claims. (Cl. 257-1) This invention relates to heat transfer equipmentand has particular although not limited reference to heat exchangedevices in which a congealable liquid and a coolant are circulated inheat transfer relation to one another to cool the liquid or to warm thecoolant or both.

An object of the invention is to achieve the desired heat transferresults without excessive congealing of the congealable liquid.

Another object of the invention is to obviate misoperation of the heatexchange device resulting from local overcooling of the congealableliquid.

A further object of the invention is to present a generally new methodfor the prevention of congealing and the obtaining of maximum heattransfer eificiency, in the cooling of a congealable fluid.

Still another object of the invention is to introduce a principle ofvariable film coefficient in heat transfer between fluids in a heatexchange core.

According to its illustrative embodiment, the invention has particularreference to the lubricant circulating and fuel supply systems ofaircraft and other vehicles which operate at times under atmosphericconditions of very low temperature, as on the order of 65 F. below zero.At extremely low temperatures of this value the fuel is in a conditionconducive to icing at the carburetor or fuel injectors. It is desirable,therefore, to warm the fuel to a more moderate temperature, and it hasbeen proposed to do this by interposing in the fuel supply line a heatexchanger comprising essentially a core of thin wall tubes with the fuelflowing through the tubes and circulating engine oil flowing over andaround the tubes. The engine oil is, of course, heated in the operationof the engine and in flowing through the described core rejects heat tothe fuel. The temperature differential between the heated engine oil andlow temperature fuel is so large that there is a tendency for theflowing oil to thicken and congeal about the tubes of the heat exchangecore with the result that the rate of heat transfer drops off materiallyand the device becomes inadequate properly to warm the fuel.

The congealing tendency can be compensated for by constructing the coreof the heat exchange device of such diameter and of such length that theamount of heat transferred is sufficient for the purpose even thoughpart of the oil is congealed. A large core, as necessitated by thisrecourse, is undesirable for a number of reasons but most importantlybecause it is costly in terms of space and weight, both of which arecritical criteria in the design of aircraft parts and accessories.

The instant invention has in view the provision of a heat exchange coreconventional in its mode of heat transfer operation but introducing agenerally new concept in the operation of such devices wherein differentparts of the core accomplish a rejection of heat from one fluid toanother according to different heat transfer coefiicients. Stillfurther, the invention has in view the making use of a further newconcept in.heat exchange devices wherein the individual tubes of thecore are 2,864,588 Patented Dec. 16, 1958 constructed to produce jetfloW-turbulizing characteristics in the fluid flowing through the tube,materially increasing the ability of the tube to transfer heat.According to the illustrative embodiment of the invention the core tubesare contsructed in part with such means for producing the jetflow-turbulizing characteristics of a fuel flowing therethrough and arein other parts constructed without such means. Thus, and further inaccordance with the illustrative embodiment of the invention, the inletends of the tubes are of a conventional cylindrical configurationwhereas the outlet ends of the tubes are constructed for increased heattransfer capabilities as above described. It is proposed, and the partsare so constructed and arranged, that during its initial passage throughthe core, the low temperature fuel receives some heat from the oil butnot at a rate sufficient to cause congealing in the oil. Continued flowof the fuel then is in a stage of high velocity, turbulent, high heattransfer efliciency in which the fuel takes on added heat from the oilat a rate designed to produce the desired fuel temperature condition. Inthis stage of the operation, however, the dilferential between theengine oil temperature and the fuel temperature is not so great asexcessively to lower the temperature of the oil.

According to the inventive concept, therefore, heat exchange apparatusis provided making use of high efiiciency heat transfer concepts and somake possible a heat exchanger of acceptably small size and low weightand yet to avoid the problem of congealing which would normally beexpected to result from use of a heat exchanger so characterized. Also,it will be understood that the invention has broadly in view theestablishing and defining of a variable film coefiicient of heattransfer and that this may be achieved by specifically difierent meansand for specifically different purposes.

Referring to the drawings,

Fig. l is a View in longitudinal section of a heat exchanger constructedin accordance with the illustrative embodiment of the invention; and

Fig. 2 is a view in longitudinal section of a heat exchange tubecomprised in the device of Fig. 1.

As shown in Fig. 1 a heat exchange device in accordance with theillustrative embodiment of the invention comprises a cylindrical openended shell 10 in one end of which is installed a closure cap 11. In theother end of the shell 10 is a fitting 12 having a central opening 13adapted to receive one end of a pipe (not shown) leading from the sourceof fuel supply. Interposed between the ends of the shell 10 andlongitudinally spaced from one another are tube sheets or header plates14 and 15 having openings therein to receive respective ends of openended thin wall heat exchange tubes 16. The plate 14 together withclosure cap 11 defines a chamber 17 at one end of the shell 10. Theplate 15 together with fitting 12 defines a chamber 18 at the other endof the shell. Fuel from the fuel source is admitted to chamber 18 by wayof opening 13, and it will be understood that the fuel is supplied undera suitable pressure. From chamber 18, the fuel flows-by way of tubes 16to the oppositely disposed chamber 17 from which it leaves the shell 10by way of a radial opening 19 therein. From the opening 19, in a mannerwhich it is unnecessary here to consider the fuel continues its path tothe engine.

Another radial opening 21 in the shell 10 provides for inflow of engineoil to the shell 10. Within the shell, the oil flows over and around theassembled core of heat exchange tubes 16 toward an outlet opening 22over which may be superimposed a fitting 23. Bafiies 24 and 25 may, asindicated, be installed in the shell to compel the oil to follow acircuitous path through the core, which path is in the main in crossflow relationship to 3 the tubes 16 and in generally counterflowrelation to the movement of the fuel. It will be understood that theseveral parts of the heat exchange device described are interconnectedin a secure, leak-proof manner, as by brazing. Further, the device willordinarily include valve drical in its configuration, The balance of thelength of the tube is interrupted by longitudinally spaced apart annularbeads 26. The described beads are short in length compared to the tubedistance between adiacent beads, and constitute longitudinallyspacedapart flow restrictors which tend intermittently to speedup the velocityof the fuel as it passes through the tubes. Further, the normal tubediameter beyond each restriction becomes in effect an expansion chamberin which the fuel flow hecomes random and turbulent andmay even" reverseitself before being forced through the next succeeding restriction torepeat the turbulizing action in the next following expansion chamber.Along the beaded parts of the tubes 16, therefore, the flowing fuel isgiven added velocity plus turbulence for an increased rate of heatexchange between the oil and the fuel. A turbulence in the engine 3 oil,produced by the externalconfiguration of the tubes in the beadedportions thereof is thought further to add to the heat exchangeefficiency. In the assembly of the core structure, the tubes 16 areplaced with the smooth cylindrical ends thereof at the inlet end of thecore, or that end facing inlet chamber 18. As a result, the relativelycold fuel enters the heat exchange core in an area in which the rate ofheat-transfer between the fuel and oil is comparatively low. In thisfirst heat exchange stage, therefore, the fuel is warmed but not at arate calculated to produce congealing of the oil around the tubes. Asflow of the fuel continues, however, it on counters the beaded portionsof the tubes and the rate of heat transfer risesrapidly, with the fuelreaching its selected temperature value as it is discharged into thechamber 17. Here again, however, the preliminary warming of the fuelprovided in the initial stage of operation is sufi'icient to prevent alarge loss of heat from the oil of the kind required for congealing ofthe oil.

Further in accordance with the illustrative embodiment of the inventionthe corestructure is one of high density with the tube diameters reducedin size as compared to 4 practices of the priorart. A greater surfacearea thus is achieved, helpful to a more rapid rate of heat transferthan has been known heretofore.

It may thus be seen that in the design of certain heat exchangerequipment it may occur that cold side fluid enters the heat exchanger ata temperature lower than the congealing temperature of the hot sidefluid. In this type of device, it is desirable, and in fact necessary,for proper operation, to preventcongealing of the hot fluid as much aspossible to maintain maximum heat exchanger efficiency. In accordancewith the instant invention, it has been demonstrated and is herewithsubmitted that a variable film coetlicient of heat transfer on the coldside may advantageously be used for the prevention of com gealin and forobtainingmaximum heat transfer efficiency, with all of the advantages oflightnessin weight and smallness of size which the latter imports.

According to the illustrative embodimentof the invention, only two typesof heat transfer surface on the cold side are here considered, thesebeing the beaded tube and the plain tube. The reciprocal of the filmcoefficient is proportional to the temperature drop through the laminarlayer of fluid adjacent to the tube wall. In operation with congealableliquids it is necessary to keep the temperature of the tube wall abovethe temperature at which congealing is promoted. According to theconcepts of the instant invention, it would be possible to maintain thetube wall at a minimum allowable temperature throughout the heatexchanger if a constantly varying film coeflicient were used. Congealingcan be prevented, however, and most of the potential efiiciency can berealized by changing the cold side coefficient a limited number oftimes, in the manner here disclosed.

A variable film co-efficient might, for example, be obtained byprogressively decreasing the spacing of the tube beads from the cold tothe hot end of the heat exchanger. Also, the tube bead spacing might beheld constant while the depth of grooving of successive beads orsuccessive groups of heads is varied. Combinations of the above effectsalso are possible.

The invention has been disclosed in its application to problems arisingin the cooling of a congealable liquid. It is, of course, adapted to thesolution of other problems wherein an exchange of heat according to avariable'film coefiicient may be desirable. In one type of air to liquidheat exchanger, for example, it is desired to lower the temperature of aheated air by passing it through tubes lilrethe tubes 16 hereof whilewater or another coolant is in contact with the external walls of thetubes. A tube beaded to a maximum depth over its full lengthwoulddeliver a maximum rate of heat transfer, bnt at a cost in pressuredrop usually greater than can be afforded. Hence a progressivelychanging film coefficient, as from a maximum at the hot end of the coreto a minimum at the cold end may advantageously be used in aninstallation such as this one.

What is claimed is:

l. A method of obtaining maximum heat transfer efliciency withoutcongealing in the bringing into heat exchange relation of a hot fluidwhich is congealable and a cold fluid which is initially at atemperature lower than the congealing temperature of the hot fluid,including the steps of providing dual heat transfer surfaces havingrespectively different heat transfer film coefiicients, arranging saidsurfaces in series relation, and putting the fluids substantially incounterflow over said surfaces, the hot fluid first encountering thesurface of higher film coefficient and flowing to the surface of lowerfilm coefficient, and the cold fluid first encountering the surface oflower film coeflicient and flowing to the surface of higher filmcoefficient.

2. A method of obtaining maximum heat transfer efficiency withoutcongealing in the bringing into heat exchange relation of a hot fiuidwhich is congealable and a cold fluid which is initially at atemperature lower than the congealing temperature of the hot fluid,including the steps of providing dual heat transfer surfaces havingrespectively different heat transfer film coefficients, and putting thefluids over said surfaces in heat transfer relation, the cold fluidfirst encountering the surface of lower film coefficient and flowing tothe surface of higher film coetficient, and the hot fluid progressingfrom one surface to the other while moving in generally cross flowrelationship to the flow of the cold fluid.

References Cited in the file of this patent UNITED STATES PATENTS804,977 Provost Nov. 21, 1905 1,297,292 Cammen Mar. 11, 1919 2,252,045Spanner Aug. 12, 1941 2,293,960 Young Aug. 25, 1942 2,506,120 Turner May2, 1950

